The relay error says it all. This is a problem I have fought with with limited success in other small enviroments (namely a mediawiki server I use at work)
What you are doing is sending email to your smtp server (Ill guess eBox's, though it could be another one on your network) then trying to relay that email from eBox's SMTP server on to the next SMTP server. This is blocked by any responsible administrator as this is exactaly how spam gets sent along. (virus puts smtp server on your computer, sends email)
The way around it is to use SMTP authentication. You will have to explore the options available to you on that. You basically need to talk to your upstream SMTP provider.
A couple more points to note. My home ISP (cox.net) blocks port 25 (smtp) explicitly, and also will not let me connect to any other smtp server other than their own.
I only know the basics here, as I havent played with the eBox mail server for exactaly these reasons the actual config is beyond me. However this should point you in the right direction.